The invention relates to a method for checking the tightness of a tank system of a vehicle.
A method of this kind is disclosed, for example, in U.S. Pat. Nos. 5,890,474 and 6,131,550.
A method of this kind is disclosed, for example, in German patent publications 196 36 431 or 198 09 384.
In these methods, air is pumped into the tank system by the pressure source. A pressure is built up in this manner in a tight tank system. The increased pressure changes the operating characteristic variable of the pressure source, that is, for example, the electrical current requirement of the pump of the pressure source is increased. The measurement of the pump current therefore defines an index for the pressure in the tank. The pump current is measured at the start of the pumping operation and after the elapse of a predetermined time interval. For a tight tank system, an increase of the current is expected because of the pressure which builds up. A fault announcement xe2x80x9clarge leakxe2x80x9d is outputted when the current increase drops below an expected pregivable index.
In contrast, a fine leak check takes place in that first pumping takes place against the reference leak of approximately 0.5 mm diameter. The reference current required for this purpose is measured. Thereafter, the tank system is pumped up so long until, for a tight tank, a current level is reached which is greater than or equal to the reference current. If this current level is not reached after a pregiven time or no positive current gradient is present any longer below this current level, then the pumping is interrupted and the reference current is measured again. If it is confirmed that this reference current still lies above the pump current level reached, then a conclusion is drawn as to a fine leak in the region of 0.5 to 1 mm.
Experiments of this method in vehicles under real environmental conditions have shown that the current of the pump can drift even for constant pressure. This drift is especially pronounced for ambient humidity. First, an increase and subsequently a slow drop-off of the current is observed. This drop-off can compensate the current increase expected from the pressure buildup. In this way, and under some circumstances, a conclusion is drawn as to a leak without one being present. Furthermore, fuel condensation effects can indicate a similar compensating action.
In view of the above, it is an object of the invention to further develop a method of the species type so that it can be used for practically all ambient conditions and ambient influences, especially for ambient humidity or moisture.
This object is solved in a method for checking the operability of a tank system of the above-described type in accordance with the invention. The basic idea of the invention is to extend the tightness check when a conclusion is drawn as to a leak in order to obtain more precise knowledge or, if required, a confirmation as to whether or if a leak is actually present. If a suspicion of a large leak or a fine leak occurs during a tightness check, then the check is not ended with a fault announcement but is extended. The tank is pumped up further until the pumping time is reliably sufficient to achieve the same pressure level for a tight tank as for pumping against the reference leak. At the end of the tank measurement, pumping against the reference leak takes place once again for a short time so that only a short time span lies between the reference measurement and the tank measurement. It is very improbable that significant current drifts occur within this time interval. Even when during the extended tightness check a drift of the tank measurement occurs (for example, the current requirement of the pump drops off), this is compensated by the comparison to the subsequent reference measurement.
In this way, the influence of moisture or other instabilities of the operating characteristic variable are no longer of any practical consequence, for example, current instabilities of the pump. Even negative gradients of the tank measurement, for example, negative current gradients, which occur because of drying of the pump or because of fuel condensation, cannot lead to an incorrect fault announcement xe2x80x9cleakxe2x80x9d because, between the renewed tank measurement and the renewed reference measurement, only a very short time interval is present in which drying effects of the pump or fuel condensations cannot really have an effect.
If, after an extended tank measurement and the subsequent immediately following renewed reference measurement, the tank measurement deviates by a pregivable value from the reference measurement (for example, the pump current for the tank measurement is less than the pump current for the reference measurement and therefore a conclusion is to be drawn as to a fault), it can be provided in an advantageous embodiment that one carries out a further tightness check including a further reference measurement as well as a further tank measurement over an extended further time interval, that is, over an extended pumping time. A fault announcement is only outputted when the further reference measurement deviates from the further tank measurement by a pregivable value after the elapse of this extended time interval.
Advantageously, the further reference measurement and the further tank measurement are carried out during a later driving cycle of the vehicle.
As mentioned, operating variables can be the pump current, that is, the current takeup of the pump of the pressure source as well as the rpm of the pump and/or the voltage applied to the pump.
When utilizing the pump current as an operating variable, the fault announcement xe2x80x9cleakxe2x80x9d is only outputted when the current, which is measured during the tank measurement, is less than the current, which is measured during the reference measurement. Stated otherwise, the tank measurement deviates from the reference measurement by a negative value of the pump current.
The reference leak can, for example, be arranged parallel to the tank system; however, it can be simulated in another embodiment by a controlled partial opening of the tank-venting valve.